1. Field of the Invention
The present invention relates to a pixel circuit having an emissive element such as an organic electroluminescence (hereinafter simply referred to as “EL”) element and to a display device in which the pixel circuits are provided in a matrix form.
2. Description of the Prior Art
Conventional organic EL panels which use an organic EL element as an emissive element are known, and much research has been directed at developing these organic EL panels. In such an organic EL panel, organic EL elements are arranged in a matrix form and the light emission of each of the organic EL elements is individually controlled to achieve a display. In particular, in an active matrix organic EL panel, because a thin film transistor (hereinafter simply referred to as “TFT”) for controlling display is provided in each pixel and the light emission from each pixel can be controlled by controlling the operation of the TFT, a highly precise display can be achieved.
FIG. 14 shows an example of a pixel circuit in an active matrix organic EL panel. A data line to which a data voltage indicating brightness of a pixel is supplied is connected to a gate of a driver TFT 12 through an n-channel switching TFT 10 having its gate connected to a gate line. In addition, one electrode of a storage capacitor 14 having the other electrode connected to a capacity power supply line is connected to the gate of the driver TFT 12. The storage capacitor 14 stores the gate voltage of the driver TFT 12.
A source of the driver TFT 12 is connected to an EL power supply and a drain of the driver TFT 12 is connected to an anode of an organic EL element 16. A cathode of the organic EL element 16 is connected to a cathode power supply.
Pixel circuits each having such a structure are arranged in a matrix form. A gate line provided for each horizontal line (row) becomes an H level at a predetermined timing and the switching TFTs 10 in the corresponding row are switched on. Because data voltages are sequentially supplied onto the data line in this state, the data voltages are supplied to and stored in the storage capacitors 14 so that these voltages are maintained even after the gate line becomes an L level.
The driver TFT 12 operates according to the voltage stored in the storage capacitor 14 and a corresponding drive current flows from the EL power supply through the organic EL element 16 to the cathode power supply, so that light is emitted from the organic EL element 16 corresponding to the data voltage.
Then, the gate lines are sequentially set to an H level so that an input video signal is sequentially supplied to corresponding pixels as a data voltage. Organic EL elements 16 arranged in a matrix form emit light based on the data voltage and a display is achieved corresponding to the video signal.
In a pixel circuit having such a structure, however, when the threshold voltages of the driver TFTs 12 in the pixel circuits arranged in a matrix form vary, the luminance of organic EL elements also varies, resulting in a problem in that the display quality is impaired. It is difficult to obtain completely identical characteristics for all TFTs in the pixel circuits in the overall display panel or to prevent variations in the threshold values for switching on and off.
Therefore, there is a desire to prevent influences, to the display, of variations in threshold values among driver TFTs.
Various techniques have been proposed for a circuit for preventing influences to variation in threshold values among TFTs (for example, PCT Patent Publication No. WO/98/48403).
In that structure, however, a circuit for compensating the variation in threshold values is required. When such a circuit is employed, the number of components in a pixel circuit is increased and there had been a problem in that the aperture ratio is reduced. When a compensation circuit is added, there also is a problem in that the peripheral circuit for driving the pixel circuit must also be changed.
The present invention therefore advantageously provides a pixel circuit in which a variation in the threshold voltages among driver transistors can be effectively compensated with a simple modification.